Abstract
Abstract. Detailed model calculations of auroral secondary and photoelectron distributions for varying conditions have been used to calculate the theoretical enhancement of incoherent scatter plasma lines. These calculations are compared with EISCAT UHF radar measurements of enhanced plasma lines from both the E and F regions, and published EISCAT VHF radar measurements. The agreement between the calculated and observed plasma line enhancements is good. The enhancement from the superthermal distribution can explain even the very strong enhancements observed in the auroral E region during aurora, as previously shown by Kirkwood et al. The model calculations are used to predict the range of conditions when enhanced plasma lines will be seen with the existing high-latitude incoherent scatter radars, including the new EISCAT Svalbard radar. It is found that the detailed structure, i.e. the gradients in the suprathermal distribution, are most important for the plasma line enhancement. The level of superthermal flux affects the enhancement only in the region of low phase energy where the number of thermal electrons is comparable to the number of suprathermal electrons and in the region of high phase energy where the suprathermal fluxes fall to such low levels that their effect becomes small compared to the collision term. To facilitate the use of the predictions for the different radars, the expected signal- to-noise ratios (SNRs) for typical plasma line enhancements have been calculated. It is found that the high-frequency radars (Søndre Strømfjord, EISCAT UHF) should observe the highest SNR, but only for rather high plasma frequencies. The VHF radars (EISCAT VHF and Svalbard) will detect enhanced plasma lines over a wider range of frequencies, but with lower SNR.
Highlights
The plasma line is a well known, but little used component of the incoherent scatter radar spectrum
The frequency shift from the transmitted signal is the frequency of the scattering Langmuir wave plus the Doppler shift caused by the bulk motion of the electron gas
In practice this difference in wave vectors leads to significant differences in offset frequency and if the Doppler shift is to be measured the dispersion relation of the Langmuir waves must be known with good accuracy (e.g. Hagfors and Lehtinen, 1981; Heinselman and Vickrey, 1992; Kofman et al, 1993)
Summary
The plasma line is a well known, but little used component of the incoherent scatter radar spectrum It is a signal scattered from high-frequency electron waves, Langmuir. Nilsson et al.: Enhanced incoherent scatter plasma lines collisions will damp the Langmuir waves toward their thermal level We have used state-of-the-art model calculations of the superthermal electron distribution for both sunlit and nighttime auroral conditions (Lummerzheim and Lilensten, 1994), and calculated the expected plasma line intensity for different radars (i.e. different transmitter frequencies) and different conditions. These are compared to measurements, to validate the results. The calculated plasma line intensities have been used to predict the actual signal-to-noise ratio (SNR) for different radars, to give a guide to when plasma line measurements should be made with the different radars
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